This invention relates generally to gas generant materials such as used to inflate automotive inflatable restraint airbag cushions and, more particularly, to the enhancement of the rate at which such materials burn or otherwise react.
Gas generating materials are useful in a variety of different contexts. One significant use for such compositions is in the operation of automotive inflatable restraint airbag cushions. It is well known to protect a vehicle occupant using a cushion or bag, e.g., an xe2x80x9cairbag cushion,xe2x80x9d that is inflated or expanded with gas when the vehicle encounters sudden deceleration, such as in the event of a collision. In such systems, the airbag cushion is normally housed in an uninflated and folded condition to minimize space requirements. Such systems typically also include one or more crash sensors mounted on or to the frame or body of the vehicle to detect sudden decelerations of the vehicle and to electronically trigger activation of the system. Upon actuation of the system, the cushion begins to be inflated in a matter of no more than a few milliseconds with gas produced or supplied by a device commonly referred to as an xe2x80x9cinflator.xe2x80x9d In practice, such an airbag cushion is desirably deployed into a location within the vehicle between the occupant and certain parts of the vehicle interior, such as a door, steering wheel, instrument panel or the like, to prevent or avoid the occupant from forcibly striking such part(s) of the vehicle interior.
Gas generant compositions commonly utilized in the inflation of automotive inflatable restraint airbag cushions have previously most typically employed or been based on sodium azide. Such sodium azide-based compositions, upon initiation, normally produce or form nitrogen gas. While the use of sodium azide and certain other azide-based gas generant materials meets current industry specifications, guidelines and standards, such use may involve or raise potential concerns such as relating to the safe and effective handling, supply and disposal of such gas generant materials.
In view thereof, significant efforts have been directed to minimizing or avoiding the use of sodium azide in automotive airbag inflators. Through such efforts, various combinations of non-azide fuels and oxidizers have been proposed for use in gas generant compositions. These non-azide fuels are generally desirably less toxic to make and use, as compared to sodium azide, and may therefore be easier to dispose of and thus, at least in part, found more acceptable by the general public. Further, non-azide fuels composed of carbon, hydrogen, nitrogen and oxygen atoms typically yield all gaseous products upon combustion. As will be appreciated by those skilled in the art, fuels with high nitrogen and hydrogen contents and a low carbon content are generally attractive for use in such inflatable restraint applications due to their relatively high gas outputs (such as measured in terms of moles of gas produced per 100 grams of gas generant material).
Most oxidizers known in the art and commonly employed in such gas generant compositions are metal salts of oxygen-bearing anions (such as nitrates, chlorates and perchlorates, for example) or metal oxides. Unfortunately, upon combustion, the metallic components of such oxidizers typically end up as a solid and thus reduce the relative gas yield realizable therefrom. Consequently, the amount of such oxidizers in a particular formulation typically affects the gas output or yield from the formulation. If oxygen is incorporated into the fuel material, however, less of such an oxidizer may be required and the gas output of the formulation can be increased.
In addition to low toxicity and high gas outputs, gas generant materials desirably are relatively inexpensive, thermally stable (i.e., desirably decompose only at temperatures greater than about 160xc2x0 C.), and have a low affinity for moisture.
In addition to the above-identified desirable properties and characteristics, gas generant materials for use in automotive inflatable restraint applications must be sufficiently reactive such that upon the proper initiation of the reaction thereof, the resulting gas producing or generating reaction occurs sufficiently rapidly such that a corresponding inflatable airbag cushion is properly inflated so as to provide desired impact protection to an associated vehicle occupant. In general, the burn rate for a gas generant composition can be represented by the equation (1), below:
rb=k(P)nxe2x80x83xe2x80x83(1) 
where,
rb=burn rate (linear)
k=constant
P=pressure
n=pressure exponent, where the pressure exponent is the slope of a linear regression line drawn through a logxe2x80x94log plot of burn rate versus pressure.
Guanidine nitrate (CH6N4O3) is a non-azide fuel with many of the above-identified desirable fuel properties and which has been widely utilized in the automotive airbag industry. For example, guanidine nitrate is commercially available, relatively low cost, non-toxic, provides excellent gas output due to a high content of nitrogen, hydrogen and oxygen and a low carbon content and has sufficient thermal stability to permit spray dry processing.
Unfortunately, guanidine nitrate suffers from a lower than may be desired burn rate. Thus, there remains a need and a demand for an azide-free gas generant material which may more effectively overcome one or more of the problems or shortcomings described above.
Commonly assigned U.S. patent application Ser. No. 09/715,459, filed Nov. 17, 2000, relates generally to gas generant compositions which desirably include or contain guanylurea nitrate (also known as dicyandiamidine and amidinourea). In particular, guanylurea nitrate advantageously has a relatively high theoretical density such as to permit a relatively high loading density for a gas generant material which contains such a fuel component. Further, guanylurea nitrate exhibits excellent thermal stability, as evidenced by guanylurea nitrate having a thermal decomposition temperature of 216xc2x0 C. In addition, guanylurea nitrate has a large negative heat of formation (i.e., xe2x88x92880 cal/gram) such as results in a cooler burning gas generant composition, as compared to an otherwise similar gas generant containing guanidine nitrate.
While the inclusion or use of guanylurea nitrate in gas generant materials can serve to avoid reliance on the inclusion or use of sodium azide or other similar azide materials while providing improved burn rates and overcoming one or more of the problems, shortcomings or limitations such as relating to cost, commercial availability, low toxicity, thermally stability and low affinity for moisture, even further improvement in the burn rate of gas generant formulations may be desired or required for particular applications.
For some inflator applications, a low gas generant formulation burn rate can be at least partially compensated for by reducing the size of the shape or form of the gas generant material such as to provide the gas generant material in a shape or form having a relatively larger reactive surface area. In practice, however, there are practical limits to the minimum size of the shape or form, such as a tablet, for example, to which gas generant materials can reproducibly be manufactured and increased burn rates may be needed for particular applications which require a higher inflator performance.
Thus, there is a need and a demand for methods or techniques for increasing the burn rate of a gas generant formulation as well as for non-azide based gas generant formulations having desirably increased or elevated burn rates.
A general object of the invention is to provide a method for increasing the burn rate of a gas generant formulation as well as an improved gas generant formulation.
A more specific objective of the invention is to overcome one or more of the problems described above.
The general object of the invention can be attained, at least in part, through a method which involves adding a quantity of at least one transition metal complex of diammonium bitetrazole to the gas generant formulation. In specific preferred embodiments, the at least one transition metal complex of diammonium bitetrazole is present in the gas generant formulation in a relative amount of at least 5 wt. % and at least 10 wt. %, respectively.
The prior art generally fails to provide as effective as may be desired methods or techniques for the raising of the burn rate of a gas generant formulation, particularly a non-azide gas generant formulation, to a level sufficient and desired for vehicular inflatable restraint system applications and in a manner practical and appropriate for such applications. Further, the prior art also generally fails to provide corresponding or associated non-azide gas generant formulations which exhibit sufficiently and effectively elevated burn rates as may be desired for such vehicular inflatable restraint system applications.
In accordance with one preferred embodiment of the invention there is comprehended a method for increasing the burn rate of a gas generant formulation and which method involves including a quantity of at least about 5 composition weight percent of a copper complex of diammonium bitetrazole having an empirical formula of CuC2H6N10 in the gas generant formulation.
The invention still further comprehends, in accordance with another preferred embodiment of the invention, a gas generant formulation which includes:
a primary fuel component selected from the group consisting of copper bis-guanyl urea dinitrate, guanidine nitrate and mixtures thereof;
a primary oxidizer component selected from the group consisting of ammonium nitrate, basic copper nitrate, copper diammine dinitrate and mixtures of ammonium nitrate and copper diammine dinitrate; and
at least one transition metal complex of diammonium bitetrazole effective to enhance the burn rate of the gas generant formulation as compared to the same gas generant formulation without inclusion of the at least one transition metal complex of diammonium bitetrazole.
As used herein, references to a specific composition, component or material as a xe2x80x9cfuelxe2x80x9d are to be understood to refer to a chemical which generally lacks sufficient oxygen to burn completely to CO2, H2O and N2.
Correspondingly, references herein to a specific composition, component or material as an xe2x80x9coxidizerxe2x80x9d are to be understood to refer to a chemical generally having more than sufficient oxygen to burn completely to CO2, H2O and N2.
Guanylurea nitrate (NH2C(NH)NHC(O)NH2.HNO3) is also commonly known as dicyandiamidine and amidinourea.
Other objects and advantages will be apparent to those skilled in the art from the following detailed description taken in conjunction with the appended claims and drawings.